Porous, wet-triggered shrinkable materials

ABSTRACT

A substrate includes a double-network polymer system including a cross-linked, covalently-bonded polymer and a reversible, partially ionicly-bonded polymer, wherein the substrate has a moisture level less than or equal to 15 percent of the total weight of the substrate, wherein the substrate is porous, and wherein the substrate includes a latent retractive force. A method for manufacturing a substrate includes producing a double-network hydrogel including a cross-linked, covalently-bonded polymer and a reversible, ionicly-bonded polymer; elongating by force the double-network hydrogel in at least one direction; treating the double-network hydrogel with an organic solvent with a volatile and water-miscible organic solvent to replace a majority of water within the double-network hydrogel; evaporating the organic solvent while the double-network hydrogel is still elongated to form a substantially-dried double-network polymer system; and releasing the force to produce the substrate.

BACKGROUND

The present disclosure is generally directed to absorbent and shrinkablematerials. In particular, the present disclosure is directed tomaterials that shrink in one dimension and expand in another dimensionwhen absorbing a liquid such as water or a bodily fluid.

Responsive materials that can potentially address many unmet consumerneeds associated with existing products are needed. New applications ofthose responsive materials can also stimulate exploration anddevelopment of emerging products beyond current categories.

Related materials can include water shrinkable fibers; however, they arenot hydrogels, they do not shrink to the same magnitude, and they do notpossess elastic properties after liquid-triggered shrinking. Previousattempts at producing responsive materials include materials such asthose described in U.S. Pat. No. 4,942,089 to Genba et al. related toshrinking fiber, water-absorbing shrinkable yarn, and other similarmaterials. Shrinking fibers that are hardly soluble in water and thatare capable of shrinking in water at 20° C. by not less than 30% in notlonger than 10 seconds are obtained, for example, by spinning, drawing,and heat-treating a carboxy-modified polyvinyl alcohol under specificconditions. Yarns made from a fiber of this kind in conjunction withnonwoven fabrics made by incorporating yarns containing such shrinkingfibers in nonwoven fabrics that are shrinkable upon absorption of waterhave been proposed for tightly fitting edge portions of disposablediapers to the thigh.

Although capable of absorbing fluids, conventional hydrogels aregenerally soft and fragile in a hydrated state and brittle and hard in adried or dehydrated state. Conventional hydrogels have poor mechanicalproperties with poor stretchability and notch-resistance.

In addition, U.S. Patent Application Publication Number 2015/038613 toSun et al. describes a hydrogel composition, but does not disclosedrying/dehydrating such a composition under stress. PCT PatentApplication Publication Number WO06132661 to Muratoglu et al. describesa hydrogel that is made “tougher” by dehydrating the hydrogel after“deforming” the hydrogel using compressive force.

As a result, there is a need to enable production of a nonwoven with theattributes described herein.

SUMMARY

Unmet needs for existing products include conformance, comfort, and theelimination of leakage. Disclosed herein is a new type of responsivematerials in different forms that can simultaneously shrink in onedimension and expand in one or more other dimensions upon contact withaqueous media and body fluids to form hydrogel materials. The materialsalso have significant absorbing capacity for water and other aqueousliquids. The materials are flexible in string, fiber, or film form.

Recently a new class of hydrogels, double-networked hydrogels, has beendeveloped with very interesting mechanical properties such as highelasticity, toughness, and notch-resistance in hydrated state. Thosematerials can be used to address unmet needs in many different fields.

This disclosure describes a substrate including a double-network polymersystem including a cross-linked, covalently-bonded polymer and areversible, partially ionicly-bonded polymer, wherein the substrate hasa moisture level less than or equal to 15 percent of the total weight ofthe substrate, wherein the substrate is porous, and wherein thesubstrate includes a latent retractive force.

In an alternate aspect, a method for manufacturing a substrate includesproducing a double-network hydrogel including a cross-linked,covalently-bonded polymer and a reversible, ionicly-bonded polymer;elongating by force the double-network hydrogel in at least onedirection; treating the double-network hydrogel with an organic solventwith a volatile and water-miscible organic solvent to replace a majorityof water within the double-network hydrogel; evaporating the organicsolvent while the double-network hydrogel is still elongated to form asubstantially-dried double-network polymer system; and releasing theforce to produce the substrate.

Objects and advantages of the disclosure are set forth below in thefollowing description, or can be learned through practice of thedisclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be more fully understood, and furtherfeatures will become apparent, when reference is made to the followingdetailed description and the accompanying drawings. The drawings aremerely representative and are not intended to limit the scope of theclaims.

FIG. 1 is an SEM photographic illustration of a cross-section of anon-porous material as described herein; and

FIG. 2 is an SEM photographic illustration of a cross-section of aporous material as described herein.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present disclosure. The drawings are representationaland are not necessarily drawn to scale. Certain proportions thereofmight be exaggerated, while others might be minimized.

DETAILED DESCRIPTION

As used herein, the terms “elastomeric” and “elastic” are usedinterchangeably and shall mean a layer, material, laminate or compositethat is generally capable of recovering its shape after deformation whenthe deforming force is removed. Specifically, when used herein,“elastic” or “elastomeric” is meant to be that property of any materialthat, upon application of a biasing force, permits the material to bestretchable to a stretched biased length that is at least about fifty(50) percent greater than its relaxed unbiased length, and that willcause the material to recover at least forty (40) percent of itselongation upon release of the stretching force. A hypothetical examplethat would satisfy this definition of an elastomeric material would be aone (1) inch sample of a material that is elongatable to at least 1.50inches and that, upon being elongated to 1.50 inches and released, willrecover to a length of less than 1.30 inches. Many elastic materials canbe stretched by much more than fifty (50) percent of their relaxedlength, and many of these will recover to substantially their originalrelaxed length upon release of the stretching force.

Reference now will be made in detail to various aspects of thedisclosure, one or more examples of which are set forth below. Eachexample is provided by way of explanation, not of limitation of thedisclosure. In fact, it will be apparent to those skilled in the artthat various modifications and variations can be made in the presentdisclosure without departing from the scope or spirit of the disclosure.For instance, features illustrated or described as part of one aspect,can be used on another aspect to yield a still further aspect. Thus itis intended that the present disclosure cover such modifications andvariations.

This disclosure describes a modification of a double-network hydrogel. Adouble-network hydrogel is a hydrogel that includes two types ofpolymers. In this case, one is a permanentlycross-linked/covalently-bonded polymer; the second is a polymer withreversible cross-linkers such as ions-ligand-based cross-linkers (theionicly-bonded polymer). Double-network hydrogels have been reported tohave superior mechanical properties such as strength, elasticity, andnotch-resistance. (See, e.g., Nature, Vol. 489, p133, 2012).

The double-network hydrogel of this disclosure is modified bystretching/stressing the double-network hydrogel while it is wet andthen, while maintaining such stretching, drying it to lower than about a10-15% moisture level. The resultant product material, a double-networkpolymer system that is not a hydrogel, remains strong and flexible whendry, but is not elastic. The cross-linked polymer of the double-networkpolymer system provides strength, whereas the ionicly-bonded polymer hashad some of its bonds broken. Without being limited with respect totheory, it is believed that breaking these bonds during drying createsstored energy in the form of a latent retractive force in the drydouble-network polymer system.

In a typical hydrogel, re-hydration leads to expansion in all threedimensions. Again, without being limited with respect to theory, it isbelieved that when the dry double-network polymer system of thisdisclosure is re-hydrated, some of the broken ionic bonds are re-formed.The double-network polymer system shrinks in one dimension (e.g., in thex-y plane), while it expands in another dimension (e.g., thez-direction, where the z-direction is perpendicular to the x-y plane).For example, a string-like sample of dry double-network polymer systemdemonstrated shrinkage in length from about 5 inches to 1 inch whenre-hydrated, while the sample also expanded in diameter. A disk-shapedsample of the dry double-network polymer system shrank in diameter butincreased in thickness.

Previous attempts to make wet-triggered shrinkable materials that becomeelastic after being hydrated used processes that are relativelycumbersome. Moreover, in general, the materials will not start to shrinkuntil a couple of minutes after wetting when all the dimensions of thematerials are more than 100 micrometers. In some cases, this time scalemight not be a problem. The response speed, however, might not be fastenough in other cases such as tightening of gaps to prevent leakage inabsorbent articles. An improved version of wet-triggered shrinkablematerials is disclosed herein, where the materials include a largenumber of micro- and nano-pores in the wet-triggered shrinkablematerials that become elastic upon hydration. In addition, a simplifiedprocess of making nonporous and porous wet-triggered shrinkablematerials is also disclosed. The porous wet-triggered shrinkablematerials include double-networked polymers and start to shrink andcomplete the shrinking process much faster than non-porous counterpartsof the same dimension and general material description.

Conventional hydrogels are generally soft and fragile in their hydratedstate and brittle and hard in a dried state. Conventional hydrogels havepoor mechanical properties along with poor stretchability andnotch-resistance. Recently a new class of hydrogels, double-networkedhydrogels, has been developed with very interesting mechanicalproperties such as high elasticity, toughness, and notch-resistance whenin a hydrated state. In this disclosure, porous wet-triggered shrinkabledouble-networked hydrogel materials are disclosed that respond towetting much faster than nonporous counterparts. In addition, asimplified process has been developed to make the porous shrinkablematerials.

In various aspects of the disclosure, a string, strand, sheet, or afiber in a dry state (with less than 10-15% water content) contains alarge number of pores. The pores can be in various sizes frommicrometers to nanometers. The pores can be open or closed, althoughopen pores are preferred.

The materials absorb water or water-containing liquid to shrink at leastin one direction and swell at least in another dimension. Upon absorbingwater or water-containing liquid, the materials become elastichydrogels. In addition, the strings, strands, sheets, or fibers absorbat least four times their weight in water. For instance, in the case ofa string made from the double-network polymer system, the string'slength becomes much shorter when wetted than it was in the original drystate when no external force is applied, whereas the diameter of thestring becomes larger at the same time upon wetting. In another example,a sheet made from the double-network polymer system can shrink in lengthand width upon wetting or hydrating while its thickness increases at thesame time.

The cross-linked polymer can be polyacrylamide, polyacrylic acid, anyother suitable polymer, or any combination of these. The reversiblecross-linker can be alginate with calcium ions, gelatin with aluminumions, any other suitable polymer, or any combination of these. In a drystate, calcium ions are not significantly cross-linked with alginate.

In one specific aspect, such a material is made of at least onecross-linked hydrogel forming polymer and at least another hydrogelforming polymer with reversible cross-linker(s) in which a significantportion of the cross-linkers (e.g., 30%) are not fully cross-linked andin a free or partially free state with the polymer in a dry state. Oneexample of the cross-linked polymer is polyacrylamide. Another exampleof the cross-linked polymers is polyacrylic acid. One example of thepolymers with reversible cross-linkers is alginate with calcium ions.Another example of the reversible cross-linked polymers is gelatin withaluminum ions. In a dry state, a significant portion of calcium ions isnot cross-linked with alginate.

Previously-reported processes to make the base materials include the useof ultraviolet (UV) light for polymerization, cross-linking and curingafter mixing all the components in a container. This process sometimesproduces materials that are fragile and are easy to tear. Presumably, UVlight can damage some of the materials during polymerization and curingprocess. The improved process employed herein uses self-generated heatto accelerate polymerization and curing for making the materials withoutusing UV light irradiation. The materials produced using this improvedprocess are more consistent in terms of strength and shrinkingperformance. By placing all the ingredients under vacuum to removeoxygen, polymerization starts to generate heat that helps to acceleratepolymerization, cross-linking, and curing. Unlike the previously-usedprocess, this improved process does not need an extended period ofcuring to obtain sufficiently-performing materials.

This new disclosure is an improved version that contains a large numberof micro- and nano-pores. This new version starts to shrink much faster(example starts to shrink 8 times faster) and completes the shrinkingprocess much faster (example finishes shrinking 3 times faster).

Potential applications of the double-network polymer system includeembedding the dry double-network polymer systems in personal careproducts, absorbent medical products, and wipers in various stringlengths or shapes. The dry double-network polymer system in a productwill change shape or tighten when wetted, potentially leading to achange in shape or appearance of such products. The positions of theembedded porous materials can vary depending upon the specific needs.The embedding methods can vary as well. Specific embedding methodsinclude adhesives-based, ultrasound-based, hot-melting-based ormechanical bonding techniques such as sewing or needle-punching.Examples of the absorbent articles include diapers, feminine pads andliners, incontinent garments.

As described further in the examples below, the present disclosureincludes manufacturing the double-networked polymer system substrates.First, the double-networked hydrogels are manufactured in a hydratedstate consistent with reported literature. The double-network hydrogelscan be manufactured in a string, a strand, a sheet, a fiber, or in anyother suitable form. After curing the double-network hydrogels, thedouble-network hydrogels are mechanically stretched or elongated in oneor two selected dimensions. The stretched materials are placed in awater-miscible volatile organic solvent such as ethanol, methanol,tetrahydrofuran, acetone, or butanone for a period of time until thematerial becomes white or opaque. The materials are taken out of thesolvent and dried to generate porous wet-triggered shrinkable materials.When the elongation force is released, the dried materials(double-network polymer systems) keep the dimensions they acquired underelongation without significant changes for a long period of time underambient conditions.

While not shown, it can be desirable to use finishing steps and/or posttreatment processes to impart selected properties to the drydouble-network polymer system. For example, chemical post treatments canbe added to the double-network polymer system at a later step, or thedouble-network polymer system can be transported to cutters, slitters,or other processing equipment for converting the double-network polymersystem into a final product. Further, patterning can be placed throughknown processes into the outer surfaces of the double-network polymersystem.

For the purposes of this disclosure, samples of double-network hydrogelswere made using polyacrylamide as the cross-linked polymer and calciumalginate as the ionicly-bonded polymer. Additional detail with respectto the preparation and performance of such double-network hydrogels canbe found in U.S. Patent Application Publication Number 2015/038613 toSun et al., which is incorporated herein by reference to the extent itdoes not conflict herewith.

EXAMPLES Materials and Procedures

1. Preparation of double network hydrogel. In one vial, 0.6 g ofalginate sodium was dissolved in 10 ml water. In another vial, 3.4 g ofacrylamide was dissolved in 12.5 ml of water. The two solutions werecombined. Then 2 mg of N,N′-Methylenebisacrylamide (MBAA) and 34 mg ofammonium persulfate, dissolved in 2 ml of water, were added. Thesolution was vacuumed for 15 minutes. 8.5 mg of tetramethylethylenediamine was dissolved in 1 ml water and was added to thevacuumed solution and mixed well. The solution was then poured into apetri dish containing 80 mg of calcium sulfate slurry in 400 microliterof water and mixed. The petri dish was placed under vacuum for one hour.The hydrogel was elastic and could be easily stretched 20 times of itsoriginal length without breaking. The stretch and relaxation could berepeated more than 50 times.

2. Preparation of water-triggered shrinkable strings. The hydrogel pieceprepared in step 1 above was cut into small strings. The strings werestretched to about 6 times their original lengths and then air-dried.The air-dried strings remained stable and were flexible for bending andmanipulation without breaking. The dry strings shrank into lengths closeto their original hydrated state within a couple of minutes upon wettingwith water or urine. For instance, a thin shrinkable string of 12 cmlong became relatively fat hydrogel of 2 cm long. In comparison,polyvinyl alcohol-based shrinkable fibers of similar size shrank lessthan 50% and they shrank slower.

3. Preparation of porous wet-triggered shrinkable strings. The hydrogelpiece prepared in step 1 above was cut into small strings. The stringswere stretched to about 6 times their original lengths and were placedinto ethanol. The transparent strings gradually became opaque and thenwhite after about ten minutes. The white strings were taken out ofethanol and air-dried or heat-dried. The porous strings remained stableand flexible for bending and manipulation without breaking. The dryporous string shrank into a length close to its original hydrated statewithin less than one minute upon wetting with water or urine. Forinstance, a thin shrinkable string of 12 cm long became relatively fathydrogel of 2 cm long.

4. Comparison for structure of porous and non-porous wet-triggeredshrinkable materials. Porous and non-porous wet-triggered shrinkablestrings were made from the same batch of base hydrogel materials withthe same diameter and length. The porous strings were white and opaqueand the non-porous strings were transparent. Cross-section SEM images ofporous strings showed a large number of pores of micrometer andnanometer sizes while the nonporous fibers did not show any pores intheir cross-sections (See FIGS. 1 and 2).

5. Comparison of shrinking kinetics. When the porous and non-porousshrinkable strings of the same dimension were placed into water, theporous shrinkable strings started to shrink much faster and complete theshrinking process to the relaxation state faster than their non-porouscounterparts. For instance, a porous wet-triggered shrinkable string of8 cm length with a weight of 20 mg started to shrink at 15 seconds afterplaced into water and completed the shrinking process to about 2 cm longelastic hydrogel around 1.5 minutes after in water. In contrast, anon-porous wet-triggered shrinkable string of 8 cm length with a weightof 20 mg started to shrink at around 2 minutes after placed into waterand completed the shrinking process to about 2 cm long elastic hydrogelaround 4.5 minutes after in water.

In a first particular aspect, a substrate includes a double-networkpolymer system including a cross-linked, covalently-bonded polymer and areversible, partially ionicly-bonded polymer, wherein the substrate hasa moisture level less than or equal to 15 percent of the total weight ofthe substrate, wherein the substrate is porous, and wherein thesubstrate includes a latent retractive force.

A second particular aspect includes the first particular aspect, whereinthe substrate is liquid absorbent.

A third particular aspect includes the first and/or second aspect,wherein the cross-linked, covalently-bonded polymer is polyacrylamide.

A fourth particular aspect includes one or more of aspects 1-3, whereinthe reversible, partially ionicly-bonded polymer is calcium alginate.

A fifth particular aspect includes one or more of aspects 1-4, whereinthe substrate is flexible and inelastic.

A sixth particular aspect includes one or more of aspects 1-5, whereinthe substrate is configured to release the retractive force when exposedto aqueous liquid.

A seventh particular aspect includes one or more of aspects 1-6, whereinthe release of the retractive force results in the substrate shrinkingin at least one dimension.

An eighth particular aspect includes one or more of aspects 1-7, whereinthe release of the retractive force results in the substrate expandingin at least one dimension that is different from the shrinkingdimension.

A ninth particular aspect includes one or more of aspects 1-8, whereinthe double-network polymer system is configured to become adouble-network hydrogel when exposed to aqueous liquid.

In a tenth particular aspect, a method for manufacturing a substrateincludes producing a double-network hydrogel including a cross-linked,covalently-bonded polymer and a reversible, ionicly-bonded polymer;elongating by force the double-network hydrogel in at least onedirection; treating the double-network hydrogel with an organic solventwith a volatile and water-miscible organic solvent to replace a majorityof water within the double-network hydrogel; evaporating the organicsolvent while the double-network hydrogel is still elongated to form asubstantially-dried double-network polymer system; and releasing theforce to produce the substrate.

An eleventh particular aspect includes the tenth particular aspect,wherein the organic solvent is ethanol.

A twelfth particular aspect includes the tenth and/or eleventh aspects,wherein elongating and evaporating captures a latent retractive force inthe substrate.

A thirteenth particular aspect includes one or more of aspects 10-12,wherein the substrate is configured to release the retractive force whenexposed to liquid.

A fourteenth particular aspect includes one or more of aspects 10-13,wherein the release of the retractive force results in the substrateshrinking in at least one dimension.

A fifteenth particular aspect includes one or more of aspects 10-14,wherein the release of the retractive force results in the substrateexpanding in at least one dimension that is different from the shrinkingdimension.

A sixteenth particular aspect includes one or more of aspects 10-15,wherein the cross-linked, covalently-bonded polymer is polyacrylamide.

A seventeenth particular aspect includes one or more of aspects 10-16,wherein the reversible, ionicly-bonded polymer is calcium alginate.

An eighteenth particular aspect includes one or more of aspects 10-17,wherein the double-network hydrogel is elastic, and wherein thesubstrate is flexible and inelastic.

A nineteenth particular aspect includes one or more of aspects 10-18,wherein the double-network polymer system is configured to return to adouble-network hydrogel when exposed to aqueous liquid.

A twentieth particular aspect includes one or more of aspects 10-19,wherein the substrate is in the form of a web, a string, a disk, asheet, or a fiber.

While the disclosure has been described in detail with respect to thespecific aspects thereof, it will be appreciated that those skilled inthe art, upon attaining an understanding of the foregoing, can readilyconceive of alterations to, variations of, and equivalents to theseaspects. Accordingly, the scope of the present disclosure should beassessed as that of the appended claims and any equivalents thereto.

1.-9. (canceled)
 10. A method for manufacturing a substrate, the methodcomprising: producing a double-network hydrogel including across-linked, covalently-bonded polymer and a reversible, ionicly-bondedpolymer; elongating by force the double-network hydrogel in at least onedirection; treating the double-network hydrogel with an organic solventwith a volatile and water-miscible organic solvent to replace a majorityof water within the double-network hydrogel; evaporating the organicsolvent while the double-network hydrogel is still elongated to form asubstantially-dried double-network polymer system; and releasing theforce to produce the substrate.
 11. The method of claim 10, wherein theorganic solvent is ethanol.
 12. The method of claim 10, whereinelongating and evaporating captures a latent retractive force in thesubstrate.
 13. The method of claim 12, wherein the substrate isconfigured to release the retractive force when exposed to liquid. 14.The method of claim 13, wherein the release of the retractive forceresults in the substrate shrinking in at least one dimension.
 15. Themethod of claim 14, wherein the release of the retractive force resultsin the substrate expanding in at least one dimension that is differentfrom the shrinking dimension.
 16. The method of claim 10, wherein thecross-linked, covalently-bonded polymer is polyacrylamide.
 17. Themethod of claim 10, wherein the reversible, ionicly-bonded polymer iscalcium alginate.
 18. The method of claim 10, wherein the double-networkhydrogel is elastic, and wherein the substrate is flexible andinelastic.
 19. The method of claim 10, wherein the double-networkpolymer system is configured to return to a double-network hydrogel whenexposed to aqueous liquid.
 20. The method of claim 10, wherein thesubstrate is in the form of a web, a string, a disk, a sheet, or afiber.